The present invention relates to gas and steam turbines and, more particularly, to apparatus for improving the cooling in vanes or buckets of turbines.
The Carnot efficiency of a heat engine is limited by, along other parameters, the maximum temperature of the working fluid fed to it. Relatively small increases in working fluid temperature can result in substantial efficiency increases. The temperature which is used is limited by the ability of materials in the apparatus to withstand the temperature and continue to function without melting or other forms of destruction. Early attempts to increase the working temperature included the use of metals having superior strength and toughness at elevated temperatures near their melting points. A limit is reached even in so-called super alloys at about twelve to fourteen hundred degrees F. beyond which the material will fail.
Gas and steam turbines represent one type of heat engine in which increasing the working temperature by a relatively small amount results in a relatively large improvement in efficiency. In a gas or a steam turbine, the working fluid (super heated steam or heated air and products of combustion) is directed against blades or buckets of one or more turbine stages to rotate the blades or buckets for delivering power to a shaft. In order to maximize the power derived from the working fluid, it is directed to the first stage turbine through nozzles which are formed between adjacent aerodynamically shaped blades which turn and accelerate the working fluid for impingement on the blades or buckets. Additional nozzles may be employed between subsequent turbine stages to accept the working fluid from the preceding stage, turn, direct and accelerate it for impingement on the next downstream stage. As the working fluid gives up energy to the turbine, it expands and its temperature reduces.
The first one or two stages of vanes forming nozzles thus receive the hottest working fluid and their ability to tolerate high temperatures provides the effective limit to the overall efficiency of the turbine.
One of the techniques employed in the prior art includes active cooling of critical parts employing cooling gas or liquid. For example, U.S. Pat. Nos. 4,244,676; 3,804,551; 4,017,210 and British Pat. No. 641,146 employ cooling flow of liquid or gas in radial passages in turbine blades. U.S. Pat. No. 3,706,508 accomplishes substantially the same result using radial passages in vanes defining turbine nozzles.
A different approach employs coring or hollowing the interior of stator vanes and flowing a cooling gas such as air therein for carrying off the heat. In order to improve the cooling still further, a sheet metal impingement insert may be inserted into the hollow core with holes or other openings directing cooling air at the inner surface of the vane for further improving of cooling. A problem may arise in such cored vanes in the aft end of the hollowed portion. Hot spots may develop on the exterior due to the fact that the cored portion is necessarily quite narrow in this region and it is difficult to properly direct and control cooling air.